Moisture indicators are used, for example, to determine the amount of moisture or humidity in the vicinity of the indicator. One type of moisture indicator changes color upon exposure to moisture or humidity. In some embodiments, colorimetric moisture indicators change color reversibly when exposed to moisture or humidity and will revert to their original color upon removal from this exposure. In other embodiments, colorimetric moisture indicators change color irreversibly so that after initial exposure to moisture or humidity the indicator remains colored and does not reversibly change back to its original color state.
Cobalt chloride has been widely used as a moisture-indicating medium. For example, the use of cobalt chloride for determining the moisture content of paper has been disclosed. Silica gel-supports that include iron (II) or iron (III) salts or with copper chloride have also been used as moisture indicators. Also lanthanide-halide based humidity indicators have been disclosed wherein an adsorbent support such as silica gel is impregnated with, for example, europium halide salts. However, cobalt and lanthanides are expensive and cobalt has some environmental concerns. Supports with iron salts or copper chloride do not show strong absorptions in the visible electromagnetic spectrum and are hard to detect.